Computer monitors are a very important output device for computer systems. Nowadays, most of the monitors are a CRT (cathode ray tube) based device in which a high voltage is generated by means of a high voltage transformer to drive electrons toward the screen for the generation of light spot on the screen. Normally, the high voltage that is generated by the transformer to be applied to the CRT is about 25 KV. The high voltage may cause damage to the electronic elements of the CRT circuit over time so that aging and deterioration of the electronic elements result in increase of the output voltage level of the transformer gradually or accidentally to a higher level.
Since the CRT generates X-ray radiation during its operation and the X-ray radiation so generated is dependent upon the high voltage applied to the CRT. FIG. 1 of the attached drawings shows a plot of the X-ray radiation generated by the CRT versus the voltage applied to the CRT. As can be seen from FIG. 1, the higher the voltage is, the stronger the X-ray will be generated by the CRT. To avoid strong radiation of X-ray during the operation of the monitor, an X-ray protection device is usually incorporated in the monitor which will be activated to shut down the power supply to the CRT when the high voltage applied to the CRT exceeds a threshold value which is usually taken as 27 KV in the monitor industry. This ensures that no excessive X-ray radiation may be generated.
FIG. 2 shows an example of the conventional X-ray protection device, wherein an AC voltage source 90 supplies a low voltage signal to input terminals of a transformer 10 which generates a high voltage at high voltage output terminals thereof. A voltage signal is taken from low voltage output terminals of the transformer 10 which is processed by means of a voltage division circuit comprised of resistors R1 and R2 and the divided voltage is applied to a Zener diode D for triggering a silicon control rectifier SCR which serves as a switch and once triggered cuts off the supply of the low voltage signal from the AC voltage source 90 to the transformer 10. A variable resistor VR is incorporated in the voltage division circuit so that the divided voltage applied to the Zener diode to control the switch SCR may be manually adjusted.
To ensure that the X-ray protection device of each monitor works, severe regulation is set up to be followed by the monitor manufacturers. FIG. 3 shows the conventional way of manually testing the X-ray protection device in the monitor manufacturing line. The process is started by setting the voltage applied to the CRT which is usually 25 KV as mentioned above, in other words, aligning the high voltage applied to the CRT to the CRT rating voltage (step a) and then manually stepping up the voltage to a level greater than the threshold to activate the X-ray protection device (step b). The voltage level that is applied to the CRT for test purpose is usually 30 KV or other suitable value. This voltage activates the X-ray protection device to shut down the power supply to the monitor CRT, if the X-ray protection device does not malfunction or break down, otherwise a calibration or repairing step (step c) has to be taken to fix the X-ray protection device. Thereafter, other manufacturing process, if needed, is carried out (step d).
Such a conventional design of the monitor X-ray protection device has some drawbacks, such as:
(1) Since the X-ray protection device is manually adjusted, error is inevitable.
(2) Since in testing the X-ray protection, the manufacturing line operator has to manually step up the voltage applied to the CRT to cause the X-ray protection device to operate, the operator may be subject to too much accumulated X-ray radiation if the operator has to continuously test a great number of monitors.
(3) The high voltage that is applied to the CRT to test the X-ray protection device may also cause damage to the parts of the monitor circuit for the circuit is subject to significant electrical stress caused by the high voltage.
(4) In turning on/off the monitor or switching between different operation modes, a transient voltage may be caused which may overshoot and thus is higher than the threshold, causing an unexpected shut-down of the power supply to the CRT. Furthermore, to avoid such a problem, the X-ray protection device may be modified in a conservative to be under-sensitive in order to avoid excessive X-ray radiation.
(5) The X-ray protection device is tested only once in the manufacturing line so that once the parts thereof fail, the device will not work properly.
(6) The variable resistor is subject to aging problem caused by oxidation by the atmosphere.
(7) The performance of the electronic elements of the X-ray protection device is usually affected thermally so that in the actual operation of the monitor, the calibration done in the manufacturing line may not be suitable and an excessive X-ray radiation may occur.
Thus, it is desirable to provide a monitor X-ray protection device which effectively overcome such problems.